In electric smelting furnaces for production of ferro alloys, ferro phosphorus, pig iron, and other products it is to day mainly used self-baking carbon electrodes. Conventional self-baking electrodes comprise a vertical arranged electrode casing normally made from steel, extending through an opening in the furnace roof or hood. The upper end of the electrode casing is open in order to allow addition of unbaked carbonaceous electrode paste which upon heating softens and melts and is thereafter baked into a solid carbon electrode due to heat evolved in the paste in the area of supply of electric operating current to the electrode. As the electrode is consumed in the furnace the electrode is lowered and new sections of casing are installed on the top of the electrode column and further unbaked electrode paste is added.
Conventional electrodes of this type are equipped with inner, vertical metallic ribs affixed to the inner surface of the electrode casing which ribs extend radially towards the centre of the electrode. When a new section of electrode casing is installed at the top of the electrode column, the ribs are welded to the ribs in the casing below in order to obtain continuous ribs in vertical direction. The ribs serve as a reinforcement for the baked electrode and to conduct electric current and heat radially into the electrode paste during the baking process. To compensate for the consumption of the electrode, the electrode is lowered downwardly into the furnace by means of electrode holding and slipping means.
When conventional electrodes of this type are used, the electrode casing and the inner ribs melt when the electrode is being consumed in the furnace. The metal content of the casing and the ribs is thus transferred to the product produced in the smelting furnace. As the electrode casing and the inner ribs usually are made from steel, such conventional self-baking electrodes can not be used for electric smelting furnaces for the production of silicon or for the production of ferro-silicon having a high silicon content, as the iron content in the produced product will become unacceptably high.
For processes where iron contaminates the product to be produced it has conventionally been used prebaked carbon electrodes which are produced in suitable lengths outside the smelting furnace and added to the top of the electrode column by means of threads or threaded nipples. It is also known to use modifications of conventional self-baking electrodes where contamination of the product in the furnaces with iron from the casing and the ribs are avoided or reduced.
Pre-baked electrodes are normally produced by forming sections of electrodes from a carbonaceous electrode paste, whereafter the formed complete electrode sections are subjected to heat treatment in order to bake the electrode paste into a solid carbon electrode. Such a method of production requires a long period of heat treatment and the temperature has to be closely regulated during heating and during cooling in order to prevent crack formation in the finished electrode length. Further, the baked electrode has to be machined in order to obtain an acceptable surface finish and to make threads in the ends of each electrode length. Prebaked electrodes produced in this way are therefore very costly.
In Norwegian patent No. 154860 it is disclosed a method for continuous production of elongated carbon bodies which bodies are cut into suitable lengths and which are used in the same way as prebaked electrodes after machining and threading. In this method electrode paste supplied to a casing is baked by lowering the casing containing unbaked electrode paste continuously or substantially continuously through a baking furnace which is heated to a temperature of between 700 and 1300.degree. C. by external supply of heat and by combustion of gases which evolve during the baking. Also this kind of electrodes have the disadvantage that they have to be machined and treaded before they can be used as electrodes in smelting furnace.
A number of modifications of the conventional self-baking electrode have been proposed in order to avoid contamination of the product produced in the smelting furnace. Thus in Norwegian patent No. 149451 it is disclosed a self-baking electrode wherein an electrode paste with a tar-based binder contained in a casing having no inner vertical ribs, is baked above the area where electric operating current is supplied to the electrode and where the casing is removed after baking of the electrode, but before the electrode reaches the area where electric operating current is supplied to the electrode. In this way a casing and rib free electrode can be produced. This kind of electrode has been used in smelting furnaces for the production of silicon, but has the disadvantage compared to conventional prebaked electrodes that it needs costly apparatuses for baking of the electrode as the electrode in the area of baking has to be heated to a temperature in the range of 700-1000.degree. C. Further, as gases containing polyaromatic hydro-carbon compounds (PAH) evolve during baking, the apparatus has to be equipped with means for collecting and destructing the PAH compounds. Finally, it has to be arranged devices for removal of the casing after the electrode has been baked.
U.S. Pat. No. 4,692,929 discloses a self-baking electrode which is useful in the production of silicon. The electrode comprises a permanent metal casing having no inner ribs and a support structure for the electrode comprising carbon fibres, where the electrode paste is baked about the support structure and where the baked electrode is held by the support structure. This electrode has the disadvantage that separate holding means have to be arranged above the top of the electrode in order to hold the electrode by means of the support structure made from carbon fibres.
U.S. Pat. No. 4,575,856 discloses a self-baking electrode having a permanent casing having no inner ribs where the electrode paste is baked about a central graphite core and where the electrode is held be the graphite core. This electrode has the same disadvantage as the electrode disclosed in U.S. Pat. No. 4,692,929, but in addition the graphite core is subjected to breakage when the electrode is subjected to horizontal forces.
The above mentioned methods for producing self-baking electrodes having no inner metal ribs all have the disadvantage the they can not be used for electrodes having a diameter above about 1.2 m without a substantial increased risk of electrode breakage. In contrast, conventional self baking electrodes may have a diameter of up to 2.0 m.
In the production of all the above mentioned types of carbon electrodes it is used a carbonaceous electrode paste comprising a particulate solid carbon material, preferably anthracite, and a tar-based binder. This electrode paste is solid at room temperature. Upon heating, the paste starts to soften at a temperature in the range of 50-150.degree. C. as the tar-based binder starts to melt at this temperature. Upon further heating to about 500.degree. C. the paste starts to bake, and a complete baking to a solid carbonaceous body takes place at a temperature above about 800.degree. C.